Adaptive cruise control (ACC) in which the speed of a target vehicle is controlled in accordance with the speed of a vehicle ahead by using a radar or the like has been adopted in commercialized products. Further, in recent years, ACC in which a curve ahead of a target vehicle is detected by using a navigation system, to thereby automatically decelerate at the curve has also been developed. In the system as described above in which a vehicle is controlled and a warning is issued to a driver on the basis of not only the traveling condition of the target vehicle but also information of a road shape and the like, there is a problem that a gap in control and warning timing occurs due to influences of a detection error in the road shape, a change in the road shape caused by construction work, a calculation error in the distance between the target vehicle and the road shape such as a curve, and the like. For this reason, a technology for measuring a road shape in real time and with high accuracy is important.
Here, there is disclosed a technology in which stationary objects are detected by using a millimeter wave radar, and only a stationary object which is useful to estimate a road shape is selected from the detected stationary objects by using previous processing results and a yaw rate sensor or a steering angle sensor, to thereby estimate a curve (see Patent Literature 1). In this regard, instead of using the previous processing results, there is an alternative method in which a plurality of road shapes are assumed, useful stationary objects are selected for each road shape, and a result for which the largest number of stationary objects are selected is selected as the stationary object useful to estimate the road shape.